1. Field of the Invention
The present invention relates generally to an electromagnetic-based arresting system for use onboard an aircraft carrier to catch and stop an incoming aircraft, and, more particularly, the present invention utilizes a low inertia induction motor/generator which allows the rapid acceleration of the spool, generator and arresting cable to match the landing speed of the aircraft before applying braking torque to ultimately stop the aircraft.
2. Description of the Background
For various remote and local applications, it is desired to land aircraft on a runway that is too short for a conventional landing, such as on an aircraft carrier or other naval vessel. Traditionally, in order to catch and stop (collectively xe2x80x9carrestxe2x80x9d) aircraft on a moving ship or other short runway, a cross-deck pendant cable is placed horizontally across the landing path of the incoming aircraft. A hook or other xe2x80x9ccatchingxe2x80x9d implement hanging below the belly of the aircraft engages the horizontal cable, and the resistance of the cable is then increased as the cable is run-out down the runway until the forward movement of the aircraft is halted. Because the aircraft is typically moving at a high rate of speed, there is a high level of stress on both the aircraft and the arrestor system cable upon impact. Therefore, the cable must be allowed to run-out to approach the speed of the landing plane or utilize some other tension reduction technique in order to reduce this stress on both the aircraft and the arrestor system.
Conventional aircraft arrestor systems, such as the Mark 7 (MK7) arresting gear presently used on the CVN 68 class of aircraft carriers, utilize a hydraulic ram/fluid system to partially compensate for the speed of the landing aircraft. Specifically, the cross-deck pendant cable is connected to two purchase cables that are ultimately wound around two wire spools with enough cable to run-out the length of the landing surface. Each length of purchase cable is run through a series of sheaves and sheave dampers in order to progressively impart a resistance on the purchase cable as it is run-out down the runway. Basically, as the landing aircraft imparts tension on the purchase cables, a ram is pulled through a viscous fluid to impart this progressive resistance to both absorb the initial shock of the aircraft and to eventually stop the aircraft""s forward momentum. As the incoming aircraft xe2x80x9chooksxe2x80x9d the cross-deck pendant and runs out the purchase cables, the sheaves and sheave dampers work to slow the aircraft.
This particular prior art system is manpower intensive for both day-to-day operation as well as in terms of maintenance. Specifically, a typical arresting gear system (there are four sets of cables on an average aircraft carrier) requires approximately 18 operators per ship. Further, key components of the system have limited service life, requiring frequent replacement, because of the large stresses imparted on the system during each landing. The amount of required maintenance is intensified because of the open-loop feedback structure of the MK7. In essence, after pre-setting certain valves based on the type and speed of the landing aircraft, there is no additional feedback or adjustment made to the purchase cable tension. Especially in off-center arrests (i.e., the plane does not impact the cross-deck pendant in the center thereby causing an unequal load distribution on each purchase cable), the actual amount of tension imparted on each purchase cable is not accurately controlled.
Moreover, the hydraulic fluid itself used in the sheave-damper system is not preferred. Specifically, this fluid is often classified as a hazardous fluid. Such fluids may leak, especially through wear, and could cause a loss of operation time for maintenance or even injury.
A second, and much more complicated prior system utilizes a mechanical means to couple the purchase cable spool to a motor/generator that is already spinning. By coupling the cable to the spinning motor/generator, the tension on the cable may be increased, and the aircraft will eventually be brought to a stop. Because of the highly complicated forces at work when coupling a running motor/generator to a running cable system, this system is expensive, less reliable, and frequently prone to error or mechanical failure.
Neither of the existing prior art systems provides a reliable aircraft arrestor system with the ability to be operated with a minimal number of personnel. Further, each of these systems are limited in the size and speed of landing aircraft that can be arrested. The present invention preferably provides better reliability through redundant control systems, reduced manning (e.g., 4-9 operators/ship) through automation, and reduced maintenance. As such, the present invention, in at least one preferred embodiment, addresses one or more of the above-described and other limitations to prior art systems.
In accordance with at least one preferred embodiment, the present invention provides an electromagnetic aircraft arrestor system utilizing a low inertia, low torque induction machine (i.e., motor/generator) to apply a progressive and controlled tension to the purchase cables during cable run-out. The basic elements of the present arrestor system may be designed to interface directly with existing sheave-damper systems currently employed with prior art hydraulic arrestor systems. One of two purchase cables are connected to each end of a cross-deck pendant which is engaged by a landing aircraft. The opposite ends of these purchase cables are preferably wound on spools coupled to the shaft of a low inertia induction machine which provides braking torque to the spool that is translated into braking tension in the cables.
As the landing aircraft moves down the runway, its mechanical energy is translated through the running purchase cables to impart rotational energy on the one or more induction machines (now running as generators) connected to the purchase cable spools. The generated electrical energy is then dissipated through the use of a braking resistor. By monitoring the rotational movement of the induction machines during cable run-out in a closed-loop feedback orientation, the proper amount of torque can be applied to the induction machine to properly slow the landing aircraft without imparting a tension in the purchase cable that exceeds the mechanical limits of the cable.
The present arrestor system may utilize a single or dual induction machine orientation to stop a landing aircraft. In a single induction machine system, both purchase cable spools are connected to the shaft of the induction machine. In the preferred system utilizing two separate induction machines, each purchase cable spool is connected to a separate machine. With this orientation, off-center aircraft arrests may be more properly controlled, as unequal torque profiles may be used on the two induction machines to account for the difference in the stresses imparted on the two purchase cables by the aircraft landing off-center.
The induction machines of the present invention may be passive and allow for the run-out of the spooled purchase cables through low torque and low inertia. However, to aid in the initial purchase cable run-out, the one or more induction machines may also optionally be capable of operating as a motor for brief periods of times. Therefore, upon initial contact between the landing aircraft and the cross-deck pendant, the induction machine(s) may unwind a specified amount of spooled cable to reduce the initial tension in the purchase cables. After a short period of time, the induction machine will return to generator mode and, through the conversion of mechanical energy to heat in a resistor, will stop the landing aircraft according to a predefined profile and closed-loop feedback.